Previous theory and research implicates participation in meaningful activity as an important factor in enhancing older adults' health-related quality of life. Consistent with this emphasis, a previous R01 grant completed by our study group demonstrated that an activity-based intervention (the Well Elderly Intervention) reduced declines in a wide variety of health-related parameters among low income, ethnically diverse elders. In the currently proposed four-year project, we aim to replicate our previous result while simultaneously examining the mediating mechanisms responsible for its positive effects. The outcomes of this study will provide important new information about the process events by which activity-based lifestyle interventions influence key aging outcomes. In the proposed study, 440 ethnically diverse elders recruited from a variety of sites in the urban Los Angeles area will participate in a randomized experiment containing a semi-crossover design component. Within either the first or second six-month phase of their study involvement, each subject will receive a lifestyle-based intervention designed to improve a variety of aging outcomes. At 4-5 points in time over an 18-24 month interval, elders will complete assessments of healthy activity, coping, social support, perceived control, stress-related biomarkers, perceived physical health, psychosocial well-being, and cognitive functioning to test the efficacy of the intervention and document the process mechanisms responsible for its effects. The study has three long-term objectives. First, it will lead to more effective health care services for our nation's rapidly growing elderly population, thereby fulfilling a major policy priority for older adults, namely, preventing declines in their health and independence. Second, it will generate new information regarding how activity influences aging outcomes. Although previous research has shown that activity patterns consistently relate to important aging outcomes, little is known about how the psychological and biological changes that stem from activities combine to promote successful aging. The proposed study will reduce this knowledge gap. Third, due to the significant ethnic diversity at the study sites, the project will produce results that generalize to minority elders. This outcome is important due to the increasing ethnic diversity of our nation's aging population.